Conventional internal combustion engines are provided with thermostat valves that generally have two or three channels with which the flow of the cooling liquid can be controlled, in particular in order to bring the internal combustion engine more quickly to operating temperature in that the flow rate of this liquid is slowed or prevented as soon as its temperature drops below a preadjusted threshold.
Such thermostat valves, as disclosed, for example, in DE 3705232 C2, are comprised usually of wax thermostatic elements in a single passage or dual passage configuration. A wax thermostatic element is comprised of a holder, of a valve seat, a closing element and a small wax pellet that is its main component. In the cold state, the closing element is seated on a seat and the cooling liquid does not flow about the closing element. When the temperature increases, the wax will expand and exert pressure on the closing element that lifts off its seat and allows the cooling liquid to flow past the closing element. As a result of their structure, the wax thermostat has a large thermal inertia thus enabling only a passive control action.
Accordingly, a first disadvantage of these thermostatic elements is their long response time which has the result that the temperature threshold of the cooling liquid that causes opening of the closing element must be low.
In order to avoid thermal problems that are correlated with a transient load of the motor, it must be ensured that the wax thermostatic element will open before the cooling liquid has reached a temperature that is too high.
As a result of fluctuations of the opening temperature threshold, the wax thermostatic element cannot provide a satisfactorily precise control action so that an optimization of the motor performance with respect to heat management is not possible with such thermostatic elements.
Up to now, the thermostats have controlled the flow through the motor radiator and an additional bypass that enables to bypass the motor radiator while in other flow passages the cooling medium flows constantly (for example, for heating the passenger compartment). This branch design slows the increase of the motor temperature and has a disadvantageous effect on motor performance and pollutant emissions during the starting phase of the engine.
For eliminating these disadvantages, it has been proposed recently to add to the wax thermostatic element an electric resistor in order to heat the wax and to thus accelerate opening of the closing element.
Such an addition increases the temperature threshold of the cooling liquid and thus the effectiveness of the control action; however, the response time of the thermostat is relatively long and the precision of the control action is in this far unsatisfactory in that the electric resistor represents only assistance for opening the closing element but not for its closure, and the wax still exhibits a strong thermal inertia.
It is also known to furnish the liquid circuits with control valves instead of with the wax thermostatic elements, in particular with control valves that are provided with rotating or linear closing elements.
Such valves must however be provided with dynamic seals, for example, shaft sealing rings, that give rise to problems in respect to management of frictional forces between valve shaft and valve receiving bore, which problems require oversizing of the actuator which then increases the required energy for the control action and leads to oversizing of the closing element, which increases the response time of the closing element.